THERMAL-KINEMATIC LINKAGES IN OROGENS

Orogenic processes operate at all scales, from the lithosphere to grain-scale, and are inexorably linked. Over the last two decades, numerical and theoretical models of collisional systems have become increasingly complex, allowing explicit coupling of fundamental processes such as kinematics, mechanics, and thermal evolution at the orogen scale. At the same time, significant advancements have been made in the comparative analysis of large multi-disciplinary, field derived datasets from a variety of orogens worldwide. To some degree, however, the progress achieved by this work is limited by our ability to reconcile what we observe in the field with model predictions. In the UK Structure and Geodynamics Group, we are coupling comprehensive field studies in a number of orogens with advanced computational modeling to test fundamental ideas in collisional tectonism.  

At the scale of an orogenic wedge, mechanical and thermal strain continuum models indicate that bulk thermal architecture is governed by the inherent kinematic asymmetry of the pro- and retro-sides of the wedge. In natural systems, these zones are arguably more complex, as major orogenic components such as the high-grade hinterland, low-grade hinterland, and exterior foreland fold-thrust belt are generally separated by discrete faults and/or shear zones. These boundaries commonly accommodate substantial thrust and in some cases normal sense displacement of 10's to 100's kms. Displacement of this magnitude can lead to juxtaposition of rocks that do not share a common protolith and, perhaps most importantly, finite deformational and thermal history. Observation of the latter indicates that these structures exert a critical influence on the thermal evolution (and by association controls deformational style through temperature-dependent rheology) of collisional systems and highlights our need to understand this thermal-kinematic linkage. Our group uses the Scottish Caledonides as a natural laboratory to investigate how crustal-scale thrust faults influence the thermal architecture of orogenic systems and how that thermal architecture controls the rheological response of the wedge. To do this, we integrate detailed field work, quantitative pressure-temperature and geochronologic analyses, and finite-element modeling. 

This study is being conducted in conjunction with collaborators Kyle Ashley (University of Texas, Austin), Richard Law (Virginia Tech), Geoff Lloyd (University of Leeds, UK), Rob Strachan (University of Portsmouth, UK), and Calvin Mako (Virginia Tech).

 Nodal temperature results of a finite-element model of a tapered thrust wedge with single fault. In this model, the maximum thrust rate is ~80 km/Myr and the thermal distribution shown is after 6.0 Myr of motion and a total lateral (right-directed) displacement of 250 km. Deformation of the isotherms is the result of very rapid thrusting, which also results in footwall heating at rates up to 160° C/Myr. Although this model yields some first-order constraints on footwall heating rates, it lacks the influence of key processes such as erosion, mechanical weakness due to heating, and isostasy, all of which are currently being developed in our group. Figure modified from Thigpen et al. (accepted) manuscript in GSA Special Publication on Linkages and Feedbacks in Orogenic Systems.

Nodal temperature results of a finite-element model of a tapered thrust wedge with single fault. In this model, the maximum thrust rate is ~80 km/Myr and the thermal distribution shown is after 6.0 Myr of motion and a total lateral (right-directed) displacement of 250 km. Deformation of the isotherms is the result of very rapid thrusting, which also results in footwall heating at rates up to 160° C/Myr. Although this model yields some first-order constraints on footwall heating rates, it lacks the influence of key processes such as erosion, mechanical weakness due to heating, and isostasy, all of which are currently being developed in our group. Figure modified from Thigpen et al. (accepted) manuscript in GSA Special Publication on Linkages and Feedbacks in Orogenic Systems.

 Rate of temperature change in simple beam models (shown in previous figure) with a maximum thrust displacement rate of ~80 km/Myr at (top) 3-4 Myr after onset of thrusting, (middle) 4-5 Myr after onset of thrusting, and (bottom) 5-6 Myr after onset of thrusting. Note that the maximum heating rates of >120° C/Myr recorded in the bottom figure are confined to the immediate footwall <1 km beneath the thrust. Also note that the spatial extent of rapid heating rates (i.e. >40° C) is much less than that observed in models with slower thrust rates. This indicates that although increasing thrust rate can increase footwall heating rate (and by association hanging wall cooling rate), the predicted spatial extent of the effect is subsequently reduced and in some cases may be difficult to separate from near fault thermal processes such as shear heating.  Figure modified from Thigpen et al. (accepted) manuscript in GSA Special Publication on Linkages and Feedbacks in Orogenic Systems.

Rate of temperature change in simple beam models (shown in previous figure) with a maximum thrust displacement rate of ~80 km/Myr at (top) 3-4 Myr after onset of thrusting, (middle) 4-5 Myr after onset of thrusting, and (bottom) 5-6 Myr after onset of thrusting. Note that the maximum heating rates of >120° C/Myr recorded in the bottom figure are confined to the immediate footwall <1 km beneath the thrust. Also note that the spatial extent of rapid heating rates (i.e. >40° C) is much less than that observed in models with slower thrust rates. This indicates that although increasing thrust rate can increase footwall heating rate (and by association hanging wall cooling rate), the predicted spatial extent of the effect is subsequently reduced and in some cases may be difficult to separate from near fault thermal processes such as shear heating. Figure modified from Thigpen et al. (accepted) manuscript in GSA Special Publication on Linkages and Feedbacks in Orogenic Systems.


coupling between upper and lower crusTal processes in an exhumed orogenic channel 

The channel flow hypothesis represents one of the most novel and perhaps controversial orogen-scale geodynamic models ever proposed to explain how large collisional systems accommodate shortening. As is common with paradigm shifting ideas such as this one, the presence of channel flow tectonics was conceptually, and often qualitatively, proposed to explain observations in a number of other collisional orogens worldwide. In the UK Structure and Geodynamics Group we are coupling field investigations in the Nepal-India Himalayas and southern Appalachians with advanced numerical modeling to test linkages between upper and lower crustal processes in regions of presumed crustal flow.